1. Field of the Invention
The present invention relates generally to an electric induction motor assembly. More specifically, the present invention concerns an electric induction motor assembly that includes an exposed bar rotor and a stator, with a punched-to-size air gap of less than 0.012 inches between the rotor and the stator.
2. Discussion of the Prior Art
Those of ordinary skill in the art will appreciate that electric induction motors typically include an air gap between the rotor and the stator, such that the rotor can freely rotate within the stator. Furthermore, electric induction motors with eight or ten poles tend to work best with rotor bars that are open or exposed to the air gap between the rotor and the stator (so-called “exposed bar” rotors). It is known that smaller air gaps generally lead to higher efficiencies, but manufacturing limitations have conventionally resulted in relatively large air gaps.
Traditionally, rotor and stator assemblies are made up of a plurality of steel laminations, which can be punched out of a lamination blank. In one example, rotor laminations have been punched out of the same material sheet as the stator laminations, with each rotor lamination being formed of material inboard of the stator lamination. In addition, the rotor lamination is punched to the desired outside dimensions of the rotor, such that no machining is required on the rotor. The minimum air gap that has been achieved with these “punched-to-size” exposed bar rotors has been 0.0125 inches.
One prior method of decreasing the size of the air gap has been to punch the rotor laminations with a larger-than-necessary diameter and a closed bar design, and then to machine the rotor diameter down to the desired size and expose the bars. Another prior method of improving efficiency has been to simply increase the size of the motor while maintaining the minimum achievable punched-to-size air gap of 0.0125 inches. Each of these options have included undesirable effects.
While machining down a larger-than-necessary rotor has been satisfactory in some respects, in that it can decrease the size of the air gap, those of ordinary skill in the art will also appreciate that machining rotors also introduces significant detrimental consequences. For example, machining the edges of the rotor laminations increases eddy current losses in the rotor by shorting out the laminations, unless there is additional treatment to remove the offending steel that is shorting to the next lamination. Such additional treatment undesirably adds process time and limits production volumes while adding cost to the manufacturing process. Alternatively, simply increasing the size of the motor can limit the use of the larger motor in many applications because of space limitations, and often a larger motor will still not match the higher efficiency provided by a smaller air gap.